1. Field of the Invention
The present invention relates to a method of manufacturing split crucible pieces of a multiple-piece graphite crucible and, more particularly, in a method of manufacturing split crucible pieces of a multiple-piece graphite crucible constituted by an integral assembly of at least two split crucible pieces and has a spherical inner bottom surface, to a mechanical cutting process of forming a spherical bottom inner surface of each split crucible piece, which constitutes the spherical bottom inner surface of the multiple-piece graphite crucible.
2. Prior Art
In the field of electric communication apparatus, substrates for semiconductor devices and integrated circuits are manufactured from single crystal silicon bodies. A single crystal silicon body is usually obtained from polycrystalline silicon material by fusing the material and causing growth of single crystal of silicon on a seed crystal, which is raised from the melted silicon bath.
This method of growing single crystal silicon is called liquid phase epitaxy or liquid pase growth method, and utilizes a phenomenon that silicon single crystal is grown on a crystal piece, i.e., seed crystal, in a predetermined orientation relation thereto as the seed crystal is raised from a melted silicon bath. Such oriented overgrowth is utilized for obtaining a silicon single crystal body grown from polycrystalline silicon material.
Such a silicon single crystal body can be manufactured by various well-known methods. Among these methods, Czochalski method (CZ method) is usually used, because it permits mass production. In this method, a graphite crucible supporting a quartz crucible inserted therein, is used for melting polycrystalline silicon material.
FIG. 1 shows, partly in section, a conventional single crystal silicon growth apparatus based on the CZ method. Referring to the Figure, reference numeral 1 designates a crucible, in which a melted silicon bath 4 is formed by melting polycrystalline silicon material. The crucible 1 has a double-wall structure having a quartz crucible 2 and a graphite crucible 3, the former being inserted in the latter.
Graphite is material suitable for melting and other high temperature operations. However, the graphite crucible 3 cannot be directly used for melting polycrystalline silicon material, because melted silicon actively reacts with graphite. For this reason, in the application of the CZ method to the manufacture of a single crystal silicon body, the quartz crucible 2 is inserted in the graphite crucible 3, and the melted silicon bath 4 is formed in the quartz crucible 3 by putting and melting polycrystalline silicon material therein.
However, the melting temperature of the silicon material is nearly the softening temperature of quartz (around 1,550.degree. C.). Therefore, the quartz crucible is possibly softened. In order to protect and support the quartz crucible 2 from the outer surface thereof, the graphite crucible 3 is formed such that its inner shape conforms or is complementary to the outer shape of the quartz crucible 2, so that the graphite crucible 3 protects the quartz crucible 2 inserted in it from the outer surface of the quartz crucible 2.
The graphite crucible 3 has its bottom held in engagement with an engagement member 5 which is also made of graphite. For engagement with the engagement member 5, the bottom of the graphite crucible 3 is formed with an engagement recess 6. The engagement member 5 is in turn held in engagement with an engagement piece 8, which is mounted on the upper end of a vertical shaft 7. The shaft 7 and the crucible 7 are thus made integral with each other.
The vertical shaft 7 is rotatable in the directions of arrows and also vertically movable. During single crystal silicon growth, the crucible 1 which is integral with the vertical shaft 7, is rotated and vertically moved in unison therewith, thereby maintaining a fixed head level of the melted silicon bath 4 in the quartz crucible 2.
A seed crystal 9 is dipped in the melted silicon bath with the head level held constant in the quartz crucible 2, and via the seed crystal (silicon single crystal) is grown to obtain a grown single crystal body 10.
For heating the crucible 1 and thermally melting silicon material, a heater 11, such as a graphite heat generator or a resistance heating coil, is disposed such as to surround the crucible 1.
Above the crucible 1, a raising shaft 12 is provided such that it is rotatable in the directions of arrows and vertically movable. The seed crystal 9 noted above, is detachably mounted on the lower end of the raising shaft 12. In the silicon single crystal growing operation, the seed crystal 9 mounted on the lower end of the raising shaft 12, is initially held in contact with the melted silicon bath 4 with the head level held constant in the crucible 1, i.e., the quartz crucible 2, and raised while being rotated in conformity with the growth of silicon single crystal. As a result, silicon single crystal is epitaxially grown on a particular crystal face of the seed crystal 9 in a predetermined orientation relation thereto. In this way, the single crystal body 10 is obtained as epitaxially grown silicon single crystal.
As described above, the graphite crucible 3 is used with the quartz crucible 2 inserted in it for manufacturing silicon single crystal by the CZ method. In this manufacture, it is desired that the graphite crucible 3 meets the following.
(a) The graphite crucible 3 is not a one-piece crucible but, as shown in FIGS. 9 and 10, a multiple-piece crucible, which is an integral assembly of two or more, for instance three as in the illustrated example, split crucible pieces 130. The split crucible pieces 130 are obtained by splitting a mass graphite material in planes containing the axis of the graphite crucible obtained as their assembly. These split crucible pieces 130 usually have a sector-like cross-sectional profile as will be described later. PA1 (b) As shown in FIG. 10, the graphite crucible which is obtained by combining the two or more split crucible pieces 130, has a spherical inner bottom surface 132, which defines a boundary zone having an arcuate cross-sectional profile between it and an inner peripheral surface 131. In other words, as shown in FIG. 9, the inner pertipheral surface 131 and the inner bottom surface 132 of the graphite crucible are continuous to each other without defining an borderline. PA1 (c) The individual split crucible pieces 130 are produced from a cylindrical mass graphite material, which is high density isotropic graphite. Therefore, there is no adequate means for bonding together such split crucible pieces. For this reason, the split crucible pieces are produced by a mechanical cutting process. A suitable method of manufacturing the split crucible pieces without substantial material loss is thus desired. PA1 a annular groove forming step of forming a blind annular groove coaxial with the work by cutting the work from an end thereof toward the other end; PA1 a work splitting step of axially splitting the work having the annular groove into two or more pieces, constituting eventual split crucible pieces, having a sector-like or polygonal cross-sectional profile perpendicular to the axis of the work; and PA1 a bottom surface forming step of cutting each eventual split crucible piece form the side thereof opposite the sector-like or polygonal profile surface side until reaching the blind annular groove by using a spherical shape cutting means having a spherical base and one or more cutters provided on the outer periphery of the base, thereby forming each split crucible piece having a cylindrical inner surface and a spherical bottom inner surface.
The above requirements (a) to (c) of the graphite crucible will now be described in greater details.
The graphite crucible is not directly used for thermally melting silicon material, but it serves to protect the quartz crucible from the outer surface thereof. However, for growing silicon single crystal the graphite crucible is made integral with the quartz crucible. In the growing operation, the outer wall surfaces of the graphite crucible is exposed to silicon vapor and other gases, which enter inevitable interstices between the close contact surfaces of the graphite and quartz crucibles. Therefore, it is necessary that, not a one-piece, but a mutiple-piece graphite crucible is fabricated by assembling together two or more split crucible pieces.
This is so because of the following reasons. The quartz crucible inserted in the graphite crucible, is softened to be in close contact with the graphite crucible inner surfaces, when it is exposed to a high temperature for melting silicon material. Therefore, subsequent cooling down the crucible would cause deformation and rupture of the outer graphite crucible due to a thermal expansion coefficient difference between graphite and quartz.
In addition, during growth of single crystal silicon via the seed crystal, during which the crucible is exposed to a high temperature so that the quartz crucible is in close contact with the inner wall surfaces of the graphite crucible, an SiC layer is generated on the graphite crucible inner wall surfaces as a result of reactions given by the following formulas (1) and (2). EQU SiO.sub.z (quartz crucible)+3C (graphite crucible) SiC+2CO (1) EQU SiO (quartz crucible)+2C (graphite crucible) SiC+CO (2)
The generation of SiC causes internal stress generation in the graphite crucible because of the great thermal expansion coefficient difference between SiC and graphite material.
The internal stress generated is the greater the greater the extent of SiC generation and, in the extreme case, cracks are generated in the graphite crucible.
For the above reasons, a one-piece graphite crucible cannot be used. Instead, a multiple-piece graphite crucible is fabricated by combining two or more split crucible pieces to solve the above problems with the plays between adjacent ones of the split crucible pieces assembled together.
As for the inner surface of the graphite crucible which comprises an integral assembly of two or more split crucible pieces, as shown in FIG. 9, the bottom inner surface 132 should be half-spherical or dish-like and smoothly continuous with the inner peripheral surface 131 without any borderline defined therewith.
In the conventional graphite crucible having a cylindrical inner peripheral surface, the bottom inner surface 131 and the inner peripheral surface 132 are not smoothly continuous with each other but define a borderline between them. Therefore, when the crucible is heated repeatedly, stress is concentrated in the borderline, and in its repeated use the crucible may be eventually broken around a borderline portion.
In addition, SiC formed on the graphite crucible tends to be concentrated in the borderline portion where the inner peripheral surface and the bottom inner surface are not smoothly continuous. With SiC concentration, the internal stress is also concentrated, so that the graphite crucible is often cracked in this portion.
Furthermore, by forming the bottom inner surface 132 to be spherical and smoothly continuous without defining any borderline, it is possible to make the dissolved oxygen content in the melted silicon bath 4 uniform and obtain a high quality grown single crystal body.
For the above reasons, the graphite crucible is usually fabricated such that, as shown in FIG. 9, the bottom inner surface 132 is spherical or dish-like and smoothly continuous to the inner peripheral surface 132 without defining any borderline portion.
As described before in detail, the graphite crucible used for the CZ method, should meet the requirements (a) to (c) mentioned above, that is, it should be a multiple-piece graphite crucible as an integral assembly of two or more split crucible pieces, in which the bottom inner surface is spherical and smoothly continuous without defining any borderline.
The fabrication of a graphite crucible having such a construction, however, requires a great deal of man-hour, and in the case of using a graphite material it is inevitable to have resort to a mechanical cutting means. The graphite material itself, however, is expensive. Particularly, graphite crucibles used for the CZ method or the like, are fabricated by using very expensive isotropic graphite material. In this viewpoint, a very efficient method of fabrication without substantial material loss is des i red.
In a further aspect, recent wafer size increase calls for graphite crucibles of increased sizes, which are not one-piece crucibles but multiple-piece crucibles obtained by assembling a plurality of split crucible pieces. To meet this end, many large-size mass graphite materials are provided, and small-size mass graphite materials are manufactured when they are required. However, the manufacture of mass graphite materials, particularly those of isotropic graphite, takes one half to one year. For this reason, even small-size graphite crucibles are inevitably cut out from large-size and expensive mass graphite materials.
In a still further aspect, in a standard method of graphite crucible fabrication, a large-size work mass graphite material is chucked in a chuck of a boring machine or the like, and bored along axis with a boring tool mounted on the boring machine while causing its rotation. Then the outer and inner surfaces of the work are trimmed to obtain a one-piece graphite crucible work, which is then axially split into a plurality of split crucible pieces. Sine the mass graphite material itself is large in size, the boring operation results in a great amount of cutting dust. The cutting dust is discarded, or may only be used as low price carbon material for steel manufacture.
In the boring operation, a substantial proportion of the expensive graphite material is thus lost as cutting dust. Particularly, graphite material used for the graphite crucible fabrication is isotropic graphite, and its piece is very high compared to graphite electrodes for refining metals and as high as 50 or more times the price of carbon material used for steel manufacture. The loss of a substantial proportion of such expensive graphite material is greatly reflected on the price of the graphite crucible. It is thus important in the graphite crucible fabrication to recover the removed material as high utility material.
If graphite material that is removed in the boring of a mass graphite material can be recovered as a high utility mass material (usually called cut-out material) instead of the cutting dust, it can be utilized for split crucible pieces of a graphite crucible of a reduced size, which is a revolutional economical merit.
In this connection, Japanese Laid-Open Utility Model Publication No. 1-117814 discloses a boring or cutting tool, which permits fabrication of a crucible from a mass graphite material by boring the same such that material loss as cutting dust is reduced and that a cut-out mass can be obtained.
This tool has a saw blade mounted on an end of a bar, which can be rotated by gripping its year end grip.
To bore a cylindrical mass graphite material with this tool, a concentric annular groove is preliminarily formed in the material from an end thereof toward the other end. Then, the bar of the tool is inserted into the groove until it reaches the bottom thereof. In this state, the saw blade at the end of the bar is caused to cut material adjacent the groove bottom by gradually turning the grip at the bar end. In this way, a cut-out mass is taken out by leaving the outer eventual crucible material.
However, the bottom inner surface of the eventual crucible material obtained with the cutting tool, is not spherical, so that the inner peripheral wall surface and the bottom inner surface are not sommothly continuous but define a borderline between them. To make such bottom inner surface to be spherical, the cutting margin on the crucible bottom inner surface is increased.